Acrylic process aids are a critical component of clear polyvinyl chloride (“PVC”) formulations. Among other properties, they increase the melt strength so that the melt does not break when pulled during processing. In general, melt strength is proportional to the molecular weight of the process aid. Unfortunately, the haziness caused by the process aid in clear PVC is also proportional to the molecular weight of the process aid because higher molecular weight polymers are more difficult to disperse, which leads to an inhomogeneous melt. In turn, a tradeoff is always required between the properties of melt strength and clarity.
Process aids to increase melt strength have been utilized in the art. For example, WO 2008/035929 A1 discloses certain PVC process aids prepared by polymerizing a PVC seed with a monomer mixture to achieve improved gelation and inhibition of fish eye and flow mark effects during calendering. The prior art does not, however, disclose a process aid according to the present invention, which achieves simultaneous improvement in melt strength and clarity.
Accordingly, there is a need to develop PVC process aids that do not suffer from the drawbacks of the prior art, namely, that provide significant improvements in melt strength while also maintaining high clarity.